FIG. 3 is a schematic view of a channel-etched TFT formed on an active matrix wiring substrate involved in a conventional liquid crystal display device. FIG. 3(a) is a plan; FIG. 3(b), a cross sectional view; and FIG. 3(c), a fragmentary cross sectional view showing its terminal. Referring to FIG. 3(b), a gate electrode 2a is formed on a transparent insulating substrate 1. A gate insulating film 3 is formed to cover the gate electrode 2a. A semiconductive layer 4 is further formed thereon so as to overlie the gate electrode 2a. Source electrode 6a and drain electrode 7 are separated from each other on the central of the semiconductive layer 4, and connected to the same semiconductive layer 4 through an interposed ohmic is etched to contact layer 5. An area of the ohmic contact layer 5 disposed between the source and drain electrodes 6a and 7 leave it only between the semiconductive layer 4 and each of the source and drain electrodes 6a and 7. Further, a passivation film 17 is formed thereon so as to cover the surface thereof. On the passivation film 17, a transparent conductive film to provide a pixel electrode 9 is connected to the drain electrode 7 interposed with a contact through-hole 11 formed through the passivation film 17.
Next, a process for preparing the active matrix wiring substrate shown in FIG. 3 will be explained below in reference to FIG. 4.
(A) A first patterning step is carried out in which a conductive layer made of Al, Mo, Cr or the like is deposited on a transparent insulating substrate made of glass or the like to a thickness of 100 to 200 nm with a sputtering apparatus, and then gate wire 2b, gate electrode 2a and gate terminal portion 2c which is to be connected to an outside signal processing substrate for displaying are formed by a photolithographic step.
(B) Next, a second patterning step is carried out in which a gate insulating film 3 made of silicon nitride or the like, a semiconductive layer 4 made of amorphous silicon and an ohmic contact layer 5 made of n+-type amorphous silicon are laminated successively in this order to a thickness of about 400 nm, about 300 nm and about 50 nm, respectively with a PCVD apparatus, and then the semiconductive layer 4 and ohmic contact layer 5 are patterned at the same time.
(C) Then, a third patterning step is carried out in which source electrode 6a, source wire 6b, drain electrode 7 and data side terminal portion 7a are formed by photolithographic processing after depositing Mo, Cr or the like to a thickness of about 150 nm with a sputtering apparatus so as to cover the gate insulating film 3 and ohmic contact layer 5. Unnecessary part of the ohmic contact layer 5 is removed which is the part except a portion that is positioned under the source and drain electrodes 6a and 7 to form a channel part of a TFT.
(D) Thereafter, a forth patterning step is carried out in which an inorganic passivation film 17 of silicon nitride is formed to a thickness of about 100 to 200 nm with a PCVD apparatus so as to cover a back channel of the TFT, source electrode 6a, source wire 6b, drain electrode 7 and the terminal portions; a contact through-hole 11 is formed for bringing the drain electrode 7 into contact with a pixel electrode 9; and unnecessary part of the passivation film 17 which is located on the data side terminal 7a portion and unnecessary parts of the gate insulating film 3 and the passivation film 17 which are located on the gate terminal 2c portion are removed.
(E) Finally, a fifth pattering step is carried out after forming a transparent conductive film which is to be changed into the pixel electrode 9 with a sputtering apparatus.
By the above explained five patterning steps, a liquid crystal display device having the active matrix wiring substrate shown in FIG. 3 whose preparing steps are greatly reduced can be prepared.
However, the above conventional liquid crystal display device (hereinafter referred as to “prior art I”) requires screening of light by means of a black matrix provided on a CF substrate in order to inhibit the leak of light from the space between the gate wire 2b and the pixel electrode 9 and from the space between the source wire 6b and the pixel electrode 9 as shown in FIG. 3(a). In order to avoid the problems concerning the accuracy in superimposing the CF substrate on the active matrix wiring substrate, the light screening region of the black matrix needs to have a large space. As a result, the aperture ratio of the liquid crystal display device becomes low. On this account, the prior art I has such a problem that the transmittance of the liquid crystal display device becomes low.
Japanese Patent Kokai-Publication JP-A-9152625(1997) (hereinafter referred as to “prior art II”) discloses, as a means of increasing the aperture ratio, a process for overlapping the pixel electrode 9 with each of the wires and thereby removing the black matrix of the CF side. FIG. 5 is a cross sectional view showing a channel protecting TFT on an active matrix wiring substrate of the prior art 2. Referring to FIG. 5, the structure of the channel protecting TFT in the active matrix wiring substrate will be explained as follows. There are a transparent insulating substrate 1 and a gate electrode 2a which is provided thereon and connected to a gate wire 2b. They are covered with a gate insulating film 3 on which a semiconductive layer 4 is provided so as to overlie the gate electrode 2a. On the central part of the semiconductive layer 4, a channel protecting layer 13 is provided. There is provided a n+-type Si layer which covers both terminals of the channel protecting layer 13 as well as a part of the semiconductive layer 4 and is divided into two pieces to provide source electrode 6a and drain electrode 7. On the outside terminal of one piece of the n+-type Si layer which is to be the source electrode 6a, transparent conductive film 14 and metallic later 15 are provided in this order to form a source wire 6b of a two-layered structure. Similarly, on the outside terminal, other piece of the n+-type Si layer which is to be drain electrode 7, transparent conductive film 14 and metallic later 15 are provided in this order. The transparent conductive film 14 is prolonged and connected to the pixel electrode 9 to form a connecting electrode. Further, there is provided an interlaminar insulating film which covers the TFT, the gate wire 2b, the source wire 6b and the connecting electrode. On the interlaminar insulating electrode, a transparent conductive film which is to be the pixel electrode 9 is provided and connected to the drain electrode 7 of the TFT by the transparent conductive film of the connecting electrode through a contact through-hole formed through the interlaminar insulating film.
These characteristic features of the liquid crystal display device of the prior art II reside in that the pixel can overlap the wires without increasing a capacity between the pixel electrode 9 and each of the wires to provide a liquid crystal display device having a large aperture ratio and thereby being capable of displaying a bright image by forming a low dielectric interlaminar insulating film thickly between the pixel electrode 9 and the source electrode 6a as well as between the pixel electrode 9 and source wire 6b. 